A digitally controlled oscillator (DCO) is an extremely common circuit element, and the DCO is widely used in many circuits (such as a phase-locked loop circuit).
The DCO controls a frequency of the oscillator by using discrete digital signals, and therefore, the frequency of the DCO is discrete. A changed step of the frequency is a function of a digital control bit. This feature of the DCO is different from frequency change continuity of a conventional analog voltage-controlled oscillator. A minimum step of the DCO directly determines quantization noise contributed by the DCO. A smaller frequency step of the DCO leads to lower quantization noise contributed by the DCO. However, if a frequency step is smaller, a larger quantity of capacitor cells (coarse adjustment/fine adjustment cells) are usually required to cover a same frequency range, more digital control bits are usually required, and more control cables are also required to transmit the digital control bits. Therefore, the control cables may be a main factor that determines a layout area, and the control cables may be even redundant to become a circuit layout design bottleneck.
A capacitor cell in a capacitor array of a conventional DCO uses temperature code for a layout, and converts binary code to temperature code by using a row-column decoder. Each capacitor cell selects a row control cable and a column control cable by using the row-column decoder, to control the capacitor cell to work or not to work (not working means disabling). Enabling and disabling of each capacitor cell is corresponding to an increase or a decrease of an oscillator frequency. In principle, there is a monotonically linear relationship between a frequency of a digitally controlled oscillator and enabling and disabling of each capacitor cell.
The capacitor array of the conventional DCO includes a fine adjustment cell array and a coarse adjustment cell array. The fine adjustment cell array includes multiple fine adjustment cells (a fine adjustment cell may also be referred to as a fine adjustment capacitor cell in some scenarios). The coarse adjustment cell array includes multiple coarse adjustment cells (a coarse adjustment cell may also be referred to as a coarse adjustment capacitor cell in some scenarios). A coarse adjustment cell has a larger frequency step, and a fine adjustment cell has a smaller frequency step. Therefore, there is a problem of carry from the fine adjustment cell to the coarse adjustment cell. However, in the capacitor array of the conventional DCO, there is usually a mismatch (Mismatch) problem during the carry from the fine adjustment cell to the coarse adjustment cell. Practice shows that the mismatch directly affects circuit performance, and sometimes, impact of the mismatch on the circuit performance reaches an extent that cannot be ignored.